The neoplastic pod gene (Np)
may be a factor for resistance to the pest Bruchus pisorum L.

Berdnikov, V.A., Trusov,
Y.A., Bogdanova, V.S.,

Kosterin, O.E., Rozov, S.M.,
Nedel'kina, S.V.

and Nikulina, Y.N.

Institute of Cytology and
Genetics

Russian Academy of Sciences

Novosibirsk 630090, Russia

The dominant gene Np (neoplastic
pod) of garden pea (Pisum sativum L.), is known to cause the
proliferation of epidermal cells outside the pods bringing about pustular-like
formations under reduced illumination, especially in a greenhouse (1, 2). The
neoplastic phenotype looks like a true disease which has no advantage to a
plant. Nevertheless, the gene turned out to be common in numerous pea forms,
including wild pea subspecies Pisum sativum syriacum (Boiss. et Noe)
Berger (= P.s. humile Boiss. et Noe) and P.s. elatius (Bieb.)
Schmahl. (1). The screening of a world pea collection of the All-Union Plant
Breeding Institute, Leningrad (VIR), as well as the analysis of data available
from the catalogue of the Weibullsholm pea collection (3), revealed a high
concentration of the Np allele in the Balkans, the Near East, and Middle
Asia (Table 1). In contrast with the data of Dodds and Matthews (1), we found a
low proportion of Np in accessions originating from Ethiopia. All seven
examined accessions of P.s. elatius and four of the five studied
accessions of P.s. syriacum displayed the neoplastic pod phenotype under
greenhouse conditions. The four studied accessions of Pisum fulvum Sibth.
et Smith, lacked this character. It seemed to be highly improbable for such a
common allele as Np, which is, moreover, characteristic to natural forms,
to be deleterious. One could suppose that the feature appearing under artificial
conditions is only a misfunction of a gene responsible for some beneficial
property of a plant.

It was reported by Vilkova et al.
(5) that certain outgrowths of the pod wall arise in response to oviposition of
pea weevil (Bruchus pisorum L.), a common pest of garden pea. These
authors stated that in accession VIR-1884 (Afghanistan) several days after
oviposition a knob started to grow just under the area of oviposition until the
eggs were removed. In 1990, we examined a number of peas grown in the VIR
Experimental Station in Krymsk, Krasnodar Region, under a heavy pressure of the
pest, and found that some of the accessions, namely, P.s. syriacum:
VIR-7335 (Tajikistan), VIR-3727 (Turkey), and VIR-2514 (Syria); P.s. elatius:
VIR-3115 (Italy); P.s. sativum: VIR-2422 (Morocco), VIR-4911 (Tibet),
and VIR-6135 (Greece); P.s. transcaucasicum (Govorov) Makasch: VIR-2376
(Georgia); P.s. asiaticum Govorov: VIR-1884 (Afghanistan), did exhibit
the described trait. The Bruchus ovipositions caused on the third to
fifth day the growth of heavy amorphic knobs 2-4 mm in size which strikingly
resembled the neoplastic pustules. After 6-8 days the outgrowths underwent
necrosis and eventually fell away thus removing the eggs. (Sometimes numerous
pustules of much smaller size appeared on the pods without any relation to
eggs). However, a great proportion of the larvae managed to hatch earlier and
penetrate into the pods, therefore, the strategy seemed not to be very
effective. The size and number of formations varied among accessions being
maximal in the P.s. syriacum forms. It was found that the seeds of
accessions VIR-7329 and VIR-2376 later turned out to be the least infested by
bruchid larvae among 30 tested accessions.

Table 1. The proportion of
accessions with allele Np in regional samples of pea accessions from the
Weibullsholm and VIR collections.

Fig. 1. The pod of an F1
hybrid between lines WL577 (np) and VIR-5797 (Np) treated with a
0.9% NaCl solution (white arrow - one can see the marks left by a glass
capillary) and with a Bruchus individual homogenised in the solution
(black arrows). The photo was taken on the fifth day after treatment. The dark
tone of the pod is due to the presence of dominant genes Pu and Pur
(determining a purple colouration of pods). The hybrid was heterozygous for
these genes. The cells of the neoplastic formations are void of anthocyanin,
thus confirming their epidermal nature that was questioned by Dodds and Matthews
(1).

These observations allowed us to
suppose that gene Np is associated with pest resistance in pea. To test
the hypothesis we conducted the following experiment. A weevil individual was
homogenized in 0.9% NaCl solution. The homogenate and the pure salt solution
were applied to pods of two pea samples grown in a greenhouse: lines WL1238 (Weibullsholm
collection), A783, and "Slow" (received from Dr N. Weeden), that do not exhibit
a neoplastic phenotype, and the F1 hybrid between line WL577 and
VIR-5797 (Crete). Line VIR-5797 has the Np gene but its expression in the
hybrid is very weak (sparse tiny pustles). It was important to use pea forms
with weak expression of the neoplastic trait in order to discriminate natural
outgrowths from induced ones. Three days after the treatment, those regions of
the pod wall in the hybrid that were contacted with the beetle homogenate gave
rise to heavy pustules (Fig. 1) while those contacted with the pure solution did
not. The effect was the same irrespective of whether the pod epidermis remained
intact or was injured by the glass capillary. No effect of treatment was
observed in peas which lacked the Np allele. This indicates that the
homogenate contains some factor which induces proliferation of epidermal cells
provided the Np allele is present. This factor evidently should also be
present in the liquid secreted by a beetle during oviposition. Investigation of
the chemical nature of the inducing substance is currently in progress.

The present results indicate that
the Np allele may provide a measure of resistance to attack by Bruchus
pisorum by conferring on the pod epidermal cells the ability to form an
outgrowth in response to oviposition. This also explains the high concentration
of the gene in southern regions, where the pressure of the pest is especially
high (4).